1. Field of the Invention
The invention relates generally to exploration and production, and more particularly, to a method and apparatus for detecting radiation phenomena to determine selected properties of earth formations and well completions.
2. Background Art
The characteristics of geological formations are of significant interest in the exploration for and production of subsurface water and mineral deposits, such as oil and gas. To that end, a variety of techniques have been developed. One technique determines the natural radioactivity of the formation by measuring the gamma rays of naturally occurring radioactive elements like Thorium, Uranium and Potassium.
In a different technique radioactive tracers are injected into the formation. The subsequent measurement of these gamma rays gives valuable information on formation fractures and other properties.
One of the better-known techniques involves irradiating the subsurface earth formations with high-energy neutrons and monitoring the resulting induced gamma rays and the scattered neutrons.
Various types of radiation sources have been used in well logging systems. For example, neutrons or gamma rays may be generated simply through the use of radioactive isotopes (which naturally decay over time), an x-ray source may be used or neutrons may be generated in an electronic device utilizing a nuclear reaction generating neutrons on demand. U.S. Pat. Nos. 3,255,353, 4,596,926, 4,698,501, 4,705,944, 4,810,459, 4,829,176, 4,879,463, 4,904,865, and 5,012,091 describe logging instruments equipped with active radiation sources and appropriate sensors.
High-energy neutrons may be generated through the controlled collision of energized particles by using a nuclear fusion reaction. Such a system is commonly referred to as an electronic neutron generator. The generation of neutrons on demand by the use on energetic particle beams allows the construction of a neutron source which emits neutrons in bursts of well-determined lengths and time sequences. One such pulsed neutron generator is described in U.S. Pat. No. 3,461,291, assigned to the present assignee. The neutron source described in the ""291 patent uses an accelerator tube in which charged particles, such as deuterium ions, are accelerated across a potential and contacted with a target element such as tritium. The reaction between the deuterium ions with the tritium target produces almost monoenergetic neutrons at an energy level of about 14 MeV. In most applications the neutrons are not emitted continuously but in short bursts of well-defined lengths and sequence of repetition. When using such a pulsed neutron generator, the formation surrounding the well logging instrument is subjected to repeated, discrete xe2x80x9cburstsxe2x80x9d of neutrons. U.S. Pat. Nos. 4,501,964, 4,883,956, 4,926,044, 4,937,446, 4,972,082, 5,434,408, 5,105,080, 5,235,185, 5,539,225, and 5,608,215 describe logging instruments equipped with neutron generators.
These pulsed neutron instruments also include one or more sensors or detectors that record numbers of neutrons, particularly epithermal energy and thermal energy, as well as gamma rays which are emitted as a result of the interaction of the neutrons with the subsurface formations and the fluids in the wellbore itself. The gamma rays may include inelastic gamma rays which are a consequence of high-energy collisions of the neutrons with atomic nuclei in the earth formations, as well as capture gamma rays emitted when low energy (thermal) neutrons are captured by susceptible atomic nuclei in the formations. Properties of the formations which may be determined as a result of measuring neutron and gamma ray phenomena include formation density, fractional volume of void or pore space in the formation (porosity), carbon/oxygen (C/O) ratios, formation lithology, and neutron capture cross section (Sigma), among other measurements. Properties which may be determined by spectral analysis of the gamma rays include concentration of various chemical elements, for example. Properties of fluids in the wellbore may also be determined from various neutron and gamma ray measurements.
Instruments which can make measurements of a plurality of the foregoing types are also described, for example, in U.S. Pat. No. 6,032,102, and in U.S. Pat. No. Re. 36,012, both assigned to the present assignee. Generally speaking, the instruments disclosed in these patents are arranged so that a pulsed neutron source therein emits a plurality of short duration neutron bursts, these being of a duration to enable relatively accurate measurement of density, through the analysis of inelastic gamma rays, and accurate measurement of porosity, through measurement of neutron xe2x80x9cslowing down timexe2x80x9d (SDT), or rate of decay of detected neutron count rate with respect to time shortly after the end of each neutron burst.
Nuclear measurements are also useable in nuclear spectroscopy techniques to obtain qualitative and quantitative information related to subsurface fluid movement. U.S. Pat. No. 5,219,518, assigned to the present assignee, describes an instrument equipped with a neutron source and sensors adapted to measure water flow through nuclear oxygen activation. Alternative techniques for subsurface fluid measurements include the use of radioactive markers or tracers to identify flow path between formations or wells. U.S. Pat. Nos. 5,049,743, 5,182,051, 5,243,190, and 5,929,437 describe the use of elements that can be made radioactive by bombardment with neutrons so their location can be determined by nuclear logging. Logging tools equipped with gamma ray detectors are particularly suited to distinguish and determine the location of trace materials.
U.S. Pat. Nos. 5,483,061 and 5,608,214 describe gamma ray spectral tools for well logging. Conventional logging instruments also provide azimuthal indication of the location of subterranean radiation sources. U.S. Pat. Nos. 4,169,979, 4,691,102, 5,021,652, 5,205,167, 5,481,105, 6,215,120 and 6,300,624 describe logging methods and apparatus for determining the azimuthal direction of subsurface gamma ray distributions.
Neutron logging measurements may be implemented in xe2x80x9cwireline loggingxe2x80x9d and in measurement-while-drilling (MWD) or logging-while-drilling (LWD) systems. Power to activate these neutron and x-ray systems generally comes from either stored energy device or via transmission from the surface. Stored energy devices are typically batteries, but may include other devices such as fuel cells. Stored energy devices provide power continuously until they become depleted. In LWD/MWD applications, surface power is typically conveyed via drilling fluid (mud) flow down the drill pipe to a turbine in the tool. In wireline tools, an armored electrical cable is used to provide power to the source and for signal communication between the tool and the surface. The instrument detectors either transmit the data uphole or temporarily store it downhole. Typically, once gathered, the data is input to one or more formation evaluation models, which are typically software programs used to evaluate the geological formation from which the data was gathered.
It is desirable to have an instrument and method for improved detection of subsurface radiation phenomena.
The invention provides a system for detecting radiation phenomena in an area surrounding a wellbore traversing an earth formation. The system comprises an elongated support member adapted for disposal within the wellbore; radiation detectors are mounted on the support member, at least one of the detectors is adapted to detect gamma ray related phenomena; and at least one of the radiation detectors is segmented to provide focused sensitivity or adapted to provide multiple radiation phenomena measurements.
The invention provides a method for detecting radiation phenomena in an area surrounding a wellbore traversing an earth formation. The method comprises disposing a support member within the wellbore, the support member having radiation detectors mounted thereon, at least one of the detectors adapted to detect gamma ray related phenomena; at least one of the radiation detectors being segmented to provide focused sensitivity or adapted to provide multiple radiation phenomena measurements; and detecting radiation phenomena with one of the radiation detectors.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.